The present invention relates to coupling pressure transmitters to an industrial process fluid. More specifically, the invention relates to a rodable coupling for such a configuration.
Industry has long used differential pressure (DP) flow measurement technologies for process monitoring and control because it is reliable, delivers accurate measurements, and leverages well understood pressure measurement instrumentation. The benefits of DP flow measurement make it the most common flow measurement technology. It is often successfully utilized in applications with less than ideal characteristics.
Process fluids that are viscous, include entrained solids, or have a tendency to solidify are examples of marginal applications where DP flow measurement may be difficult to implement. Refinery process fluid measurements are particularly challenging. The fluid flowing in these applications is highly viscous when not heated to very high temperatures. Impulse tubes both carry the pressure induced across a primary element and reduce fluid temperature to an acceptable level for differential pressure measurement. These fluids can plug the impulse tube and cause the pressure signal to the transmitter to be attenuated or eliminated. A plugged impulse tube is difficult to diagnose and may go unnoticed by the customer resulting in measurement problems and lost revenue.
Operators address these issues in difficult applications by “rodding-out” impulse tubes. This is accomplished by opening a port at the end of an impulse tube and inserting a tool such as a metal rod or drill point. The rod is forced through any obstruction thereby clearing the impulse tube. Traditional orifice plate installations that allow this practice are commonly referred to as being “rodable”.
Some flow meter configurations are not rodable for several reasons. The impulse tubes between the head and the sensor may be constructed of small diameter tubes which are too small for robust tools to fit inside them. Additionally, the thin tubes may not be straight, and may follow a curved path between the head and the process connection. Further, the transmitter may be mounted on top of the impulse tubes thereby eliminating the possibility of mounting the fittings that would allow access for the rodding tools.